This invention relates generally to a method for forming a barrier layer to limit percolation through a surface cover over a waste site. More particularly, the invention relates to a method for forming an anisotropic capillary barrier to limit downward movement of water through the surface cover and into a waste site.
Surface covers, hereafter referred to as covers, are required by the Environmental Protection Agency (EPA) as part of an engineered structure for enclosing waste sites, landfills, mine tailing sites, surface impoundments or other environmental hazards. Surface covers perform numerous functions, including: preventing vectors, odors, and blowing litter; allowing loaded vehicles to pass over waste sites; ensuring slope stability; providing a means for controlling slope erosion; limiting percolation of surface water into the waste site; and controlling upward gas movement out of the waste site. The principal purpose of any surface cover is to limit downward movement of water through the waste site and subsequent mobilization of hazardous constituents into the surrounding ground water. The cover can vary from a simple soil structure to a complex multiple layer structure combining earthen materials and geosynthetics. The technology or combination of technologies incorporated into the cover structure are highly dependent on the waste site contents and the associated risks to the surrounding environment.
A typical surface cover structure normally includes at least one layer having a low saturated hydraulic conductivity to limit water movement through the cover system when the layer is completely wet or saturated. This layer is often referred to as a barrier layer and is the principal design feature in most cover designs. In the U.S., this approach is motivated by regulations mandated by the EPA which specify the saturated hydraulic conductivity for the barrier layer. Alternative technologies could be used instead of traditional barrier layers but these alternative technologies must meet EPA specifications. Some of the traditional or conventional barrier layers are described below:
Compacted soil layers (CSLs)--A compacted soil layer is a layer of silty or clay-like soil which is compacted or densified to induce a low saturated hydraulic conductivity. The CSL is a principal feature of the EPA-recommended cover design and has been widely applied to many waste sites throughout the U.S. As experience and information accumulates about CSLs, their shortcomings are becoming more evident. Factors affecting the integrity of CSLs include freeze-thaw effects, shrink-swell effects, erosion, subsidence, root intrusion and animal intrusion. In particular, desiccation, or drying of the soil, is a critical factor affecting the performance of CSLs. Once desiccated, CSLs are likely to crack and become ineffective as a barrier and will allow surface water to penetrate the waste site. Compacted soil layers are unlikely by themselves to be effective long-term barriers.
Geosynthetic clay liners--Geosynthetic clay liners (GCLs) are replacing compacted soil layers for some applications. A GCL is a factory manufactured hydraulic barrier consisting of a thin layer of bentonite clay supported by geotextiles and/or geomembranes. The low saturated hydraulic conductivity of bentonite allows a 5-mm thick layer of betonite used in a GCL to have an effective saturated hydraulic conductivity as low as the saturated hydraulic conductivity of a 1-m thick compacted soil layer. Many of the advantages associated with the use of GCLs as compared to CSLs are found in the GCL installation process: GCLs can be installed fast, with lightweight equipment, less field testing is required and GCLs are installed dry. Although the bentonite layer will desiccate, a low saturated hydraulic conductivity is recovered when the betonite is rewetted. The principal disadvantages of GCLs are cost, the lack of installation experience, the vulnerability of the GCLs to puncture, the long term durability of GCLs, and slope stability.
Geomembranes--Geomembranes are typically constructed of polyethylene and are usually 20 mils thick at a minimum. Although they can be installed relatively quickly, geomembranes often develop flaws during construction such as tears, punctures, and open seams. Current estimates are that more than half of the geomembranes installed have 8 or more flaws per acre. For example, although they can accommodate significant deformations, large amounts of subsidence and the settlement seen in many landfills will cause the geomembrane to fail. Another concern is the potential for the geomembrane to become brittle over time, usually caused by a variety of degradation mechanisms. Geomembranes are being used more frequently in covers as part of a composite structure that includes either a CSL or GCL. A barrier layer employing the composite structure is proving more effective in reducing or limiting downward water movement through the waste site. Enhanced functionality of the barrier layer can only be achieved if there is good contact between the geomembrane and the underlying layer.
Most of the time the cover and the area around the cover is not saturated, yet the conventional barrier layers, as described above, are designed to function under saturated conditions. In other words, these barrier layers are designed to restrict water movement when the layers are completely wet. However, in dry climates like those found in the majority of the western U.S., conventional barriers layers will not work well because of the unsaturated conditions typically found in the soils. Accordingly, simpler, less expensive cover designs more consistent with the natural environment are actively being sought.
Alternative surface cover designs are also motivated by the cost and limitations of the conventional barrier layers or composite barrier layers described above. Another reason for alternative barrier technologies is that most barrier layers degrade over time. Finally, multi-component engineered covers are expensive, and alternative designs could reduce costs.
One such alternative design to a traditional barrier layer design is a capillary barrier. Generally, a capillary barrier consists of a top layer-sublayer structure. Typically, the top layer and the sublayer are both soil layers, but the top layer is a finer granular sand-like material and the sublayer is a coarser material, typically a gravel-like material. The contrasting properties of the materials in the top layer-sublayer arrangement serve as a barrier to downward water flow. Water is preferentially held in the top layer by capillary forces as a consequence of its fine structure until the water is removed by evaporation or plant transpiration or, if the top layer-sublayer interface is sloped, water in the top layer will also drain laterally, parallel to the interface, by unsaturated flow. The measure of capillary forces within a soil is the matric potential, or pressure potential, or soil suction. The effectiveness of a capillary barrier is defined by its divergence length.
Divergence length is the distance which water is laterally diverted along the top layer-sublayer interface before any of the water moves into the sublayer. The top layer has to remain relatively dry or the capillary forces will decrease the extent to which water will drain quickly through the top layer into the sublayer. A capillary barrier is effective if the combined effect of evaporation, transpiration, and lateral divergence exceeds the infiltration from precipitation, thereby keeping the system sufficiently dry so that appreciable movement of water from the top layer into the sublayer, termed breakthrough, does not occur. Consequently, capillary barriers are thought to be most applicable to relatively arid sites such as a large portion of the western U.S.
Field studies indicate that under the most stressful conditions (high precipitation, low evapotranspiration), the divergence length of a capillary barrier having a sloped interface from 1 to 10%, is less than 10 m. A cover incorporating a capillary barrier having a divergence length of 10 m would not be sufficient to divert downward flowing water across most waste sites. Because of the relatively short characteristic divergence length of a capillary barrier, movement of water into a waste site is a real threat.
The subject invention as described herein, anisotropic capillary barriers, have greater divergence lengths than conventional capillary barriers. The increased divergence length of an anisotropic capillary barrier will make its acceptance and use applicable to more waste sites under a wider range of conditions.
Anisotropic capillary barriers have a similar structure to capillary barriers, a top layer-sublayer structure. The primary difference between an anisotropic capillary barrier and a conventional (isotropic) capillary barrier is the anisotropic unsaturated hydraulic conductivity of the top layer in the anisotropic capillary barrier. Anisotropic unsaturated hydraulic conductivity is defined by the condition that the lateral unsaturated hydraulic conductivity does not equal that of the normal unsaturated hydraulic conductivity in a given layer. The lateral direction is the direction parallel to the top layer-sublayer interface, and the normal direction is the direction perpendicular to the top layer - sublayer interface. In conventional capillary barriers the top layer has the same unsaturated hydraulic conductivity in both the lateral and normal directions. More specifically, the top layer of the anisotropic capillary barrier has a greater unsaturated hydraulic conductivity in the lateral direction than the normal direction. The lateral direction is the direction parallel to the top layer-sublayer interface, and the normal direction is the direction perpendicular to the top layer - sublayer interface. The increased divergence length found in an anisotropic capillary barrier can be attributed to the anisotropic hydraulic conductivity.
Anisotropic hydraulic conductivity can be engineered into a barrier by compaction techniques, layering and/or amending the top layer to create a composite top layer structure. The amount of anisotropy can be defined as the ratio of hydraulic conductivities in the lateral direction to the normal direction, and anisotropy can range from a factor of one (no anisotropy) to many orders of magnitude. The increased divergence length seen in anisotropic capillary barriers is approximately proportional to the ratio of the principal hydraulic conductivities for many assumed infiltration rates and material properties. The amount of anisotropy required depends on the risks associated with the specific waste site and local conditions, and can be designed for a particular situation.